Waste heat steam generating cycle

ABSTRACT

A WASTE HEAT POWER RECOVERY CYCLE IS DESCRIBED WHICH CONVERTS TO SHAFT WORK A LARGER PORTION OF THE HEAT AVAILABLE FROM A GIVEN SOURCE OF WASTE HEAT THAN DOES AN ORDNARY FRANKIE CYCLE. THE RECOVERY CYCLE IS PARTICULARLY ILLUSTRATED AS APPLIED TO THE WASTE HEAT CONTAINED IN THE EXHAUST GASES OF A BRAYTON CYCLE COMBUSTION GAS TURBINE. THE EXHAUSE GASES, AVAILABLE USUALLY AT ABOUT 850* F., HEAT 1500 P.S.I.A. PRESSURIZED WATER TO ABOUT 596* F . THE HOT PRESSURIZED WATER IS THEN INTRODUCED INTO ONE OF A PLURALITY OF WATER RECEIVERS IN WHICH IT IS PERMITTED TO FLASH AT GRADUALLY DECREASING PRESSURES. THE RESULTING FLASH STEAM IS SUPERHEATED TO ABOUT 800* F. BY THE 850* F. EXHAUST GASES, AND INTRODUCED INTO THE STEAM CHEST OF A MULTIPLE NOZZLE STEAM TURBINE, THE NUMBER OF NOZZLES PASSING STEAM IS GRADUALLY INCREASED AS THE STEAM PRESSURE DROPS SO AS TO KEEP POWER PRODUCTION SUBSTANTIALLY CONSTANT. WHEN THE STEAM PRESSURE HAS DROPPED A PREDETERMINED AMOUNT, THE UNFLASHED WATER IS TRANSFERRED TO ANOTHER RECEIVER AND IS PERNITTED FLASH DOWH TO A LOWER PRESSURE. THIS FLASH STEAM IS SIMILARLY SUPERHEATED AND IS INTRODUCED INTO THE STEAM CHEST OF A SIMILAR MULTIPLE NOZZLE INTERMEDIATE PRESSURE TURBINE. THE WATER IS FINALLY INTRODUCED INTO LOW PRESSURE WATER RECEIVE WHERE IT IS FLASHED TO ABOUT CONDENSING PRESSURE, THE FLASH STEAM IS AGAIN SUPERHEATED, AND EXPANDED THROUGH A MULTIPLE NOZZLE LOW PRESSURE TURBINE TO THE CONDENSER THE RESIDUAL WATER AND CONDENSATE ARE THEN JOINED, AND PUMPED AT HIGH PRESSURE TO BE HEATED AGAIN BY THE EXHAUST GASES AS DESCRIBED ABOVE. WHEN THE PRESSURES OF THE RECEIVERS IN THE FIRST LINE DROP TO THE PRESELECTED LOWER PRESSURES, A FRESH LINE OF RECEIVERS AT THE HIGHER PRESSURES IS CUT IN, THUS PROVIDING CONTINUOUS OPERATION.

Oct. 12, 1971 .W. H. EBGEN 1,718

WASTE HEAT STEAM GENERATING CYCLE Filed may 5, 1.970

INVENTOR. WILL/AM NEBGE/V A TTORNEY ILLS. Cl. Gil-39.18 B 3 Claims ABSTRACT OF THE DISCLOSURE A Waste heat power recovery cycle is described which converts to shaft work a larger portion of the heat available from a given source of waste heat than does an ordinary Rankine cycle.

The recovery cycle is particularly illustrated as applied to the waste heat contained in the exhaust gases of a Brayton cycle combustion gas turbine.

The exhause gases, available usually at about 850 F., heat 1500 p.s.i.a. pressurized water to about 596 F. The hot pressurized water is then introduced into one of a plurality of water receivers in which it is permitted to flash at gradually decreasing pressures. The resulting flash steam is superheated to about 800 F. by the 850 F. exhaust gases, and introduced into the steam chest of a multiple nozzle steam turbine. The number of nozzles passing steam is gradually increased as the steam pressure drops so as to keep power production substantially constant. When the steam pressure has dropped a predetermined amount, the unflashed water is transferred to another receiver and is permitted to flash down to a lower pressure. This flash steam is similarly superheated and is introduced into the steam chest of a similar multiple nozzle intermediate pressure turbine. The water is finally introduced into a low pressure water receiver where it is flashed to about condensing pressure, the flash steam is again superheated, and expanded through a multiple nozzle low pressure turbine to the condenser.

The residual water and condensate are then joined, and pumped at high pressure to be heated again by the exhaust gases as described above. When the pressures of the receivers in the first line drop to the preselected lower pressures, a fresh line of receivers at the higher pressures is cut in, thus providing continuous operation.

BACKGROUND OF THE INVENTION If its exhaust heat were efficiently utilized, a Brayton gas turbine cycle would be a very efficient power producer, because of its ability to employ a high inlet temperature. However, the inefficiency of the exhause heat conversion systems currently in use reduce this potential efliciency to the point where the overall cycle efliciency is usually even less than that of a conventional steam, or Rankine cycle.

In the past, in an attempt to improve overall efficiency, waste heat boilers have frequently been employed, and recently two waste heat boilers in series, producing steam at successively lower pressures have been used.

A thermal efficiency of. 35 .1 is theoretically attainable by a cycle operating with a waste heat source whose initial 850 F. temperature declines to 100 F. This efliciency could be reached by using a very large number of boilers in series, operating at progressively lower pressures, but in practice the cost of the equipment and parasitic losses such as pressure and temperature drop limit the number to two or at most three boilers in series.

SUMMARY OF THE INVENTION The present invention consists of a waste heat boiler system in which the sensible heat, particularly sensible heat from a Brayton cycle gas turbine, is used first to 3,011,718 Patented Oct. 12, I971 superheat the vapor of a working fluid, as will be described below, and is then used to heat the working fluid in the liquid form at a pressure sufliciently high substantially to prevent vaporization. Essentially the present invention is not concerned in its broadest aspect with the exact working fluid concerned. Water is by far the cheapest and in many cases the preferred working fluid. However, other known working fluids, such as suitable fluoro carbons, for example the one referred to by the trade designation Freon R11, ammonia and the like. Other working fluids, of course, may be used, bearing in mind of course that the fluid chosen must have vaporizing characteristics suitable for the use to which the waste heat developed energy is to be put. In the case of a Brayton cycle gas turbine, the normal exhaust gas is of the order of 800 to 900 F., and this sets certain limits on the nature of the working fluid. For example, in such a combination mercury would not be particularly desirable. While the present invention has no upper limit, other than that dictated by materials of construction, on the temperature of the waste heat source, the increase in efliciency is much more marked where the waste heat source temperature is not extremely high. In the latter case, conventional Rankine cycle waste heat boilers are often sufliciently eflicient thermally so that the need for the present invention is not as great.

In the range of temperatures represented by gas turbine exhausts, water is a very suitable working fluid and its cheapness and other characteristics makes it preferred. In order not to render the specification unclear, the further descriptions will be in terms of water as the Working fluid, without, however, limiting the invention in its broadest aspects to this particular material.

When utilizing waste heat of the temperature range of 800 to 900 F., water at a pressure of. 1500 p.s.i.a. can be heated without vaporization to a temperature of 596 'F. This hot, high pressure water is introduced into the high pressure receiver of one line of a plurality of lines of receivers, for example, two or three, each line consisting of a high, an intermediate, and a low pressure receiver. The water in the high pressure receiver is permitted to gradually flash down to an intermediate pressure, and the steam evolved is superheated by the exhaust gases, as has been referred to above, and then expanded in a high pressure steam turbine with multiple nozzles. The rate of flashing is controlled by the number of turbine nozzles in use; at the highest pressure a single nozzle may be used, while as the pressure drops and the weight of steam passed by an individual nozzle also drops, more nozzles are successively cut in to keep the power output reasonably constant.

When the water in the high pressure receiver has flashed down to a preselected lower pressure, it is transferred to an intermediate pressure receiver and the empty high pressure receiver is recharged with hot water. The water in the intermediate pressure receiver is permitted to flash as before to a lower pressure. The evolved steam joins the high pressure turbine exhaust steam, and after being superheated the combined steam is introduced into a multiple nozzle intermediate pressure turbine.

When the water in the intermediate pressure receiver has flashed down to a preselected lower pressure, it is transferred to a low pressure receiver. The empty intermediate pressure receiver is then recharged with the water which had previously flashed in the high pressure receiver. The water in the lower pressure receiver is permitted to flash as before to condensing pressure. The evolved steam joins the intermediate pressure turbine exhaust steam and after being superheated, the combined stream is introduced into a multiple nozzle low pressure turbine, through which it expands to the condenser.

When the water in the low pressure receiver has flashed down to a preselected pressure, the receiver is emptied and then recharged with the water which had previously flashed in the intermediate pressure receiver.

The flashed water from the low pressure receiver and the condensate are joined and pumped back for reheating by the exhaust gases.

Since the pressure of the steam generated by the flashing of a batch of hot water continuously declines from the preinitial pressure of 1500 'p.s.i.a. to the preselected condensing pressure, each batch of hot water provides as it vaporizes the equivalent of an infinite number of boilers operating at successively lower pressures. The cycle efficiency can, therefore, closely approach the previously mentioned maximum theoretical efficiency of 35.1%.

Neglecting heat and friction losses, the present invention will produce from a given quantity of 850 F. exhaust gases, 54% more work than a single waste heat boiler, more work than two waste heat boilers in series, and 23% more work than three boilers in series.

The vaporization of the water in the high, intermediate, and lower pressure receivers has been described as though it occurs sequentially, but it should be understood that in practice the water in all three receivers is vaporizing simultaneously, supplying steam to the respective turbines. When the preselected lower pressure has been reached in all three receivers, the water is pumped out of the low pressure receiver, the water in the intermediate pressure receiver is displaced into the low pressure re ceiver, the water in the high pressure receiver is displaced to the intermediate pressure receiver, and the high pressure receiver is supplied with a fresh charge of hot water.

The dead spot in power production which could occur While the batches are being transferred from receiver to receiver can be eliminated by employing an alternately operating line of three receivers feeding the same three turbines.

The smoothness of the flow of power from the waste heat boilers can be assured by using a hot water surge tank to which the pressurized hot water flows before entering the high pressure water receiver, and a cold water surge tank to which the cooled, flashed water is pumped from the low pressure water receiver and from the condenser. The holding capacities of the surge tanks must be sufficient to balance the cyclic water requirements of the receiver lines with the uniform water flow rate of the waste heat recovery system.

It will be apparent that the number of receivers required per line is determined both by the number of nozzles available in a given turbine and by the steam pressure ratio through which the turbine must perform.

Since the water in the low pressure receiver is always cooler than the water in the intermediate pressure receiver, which, in turn, is always cooler than the water in the high pressure receiver, any vapor space remaining in the receivers when the water is transferred will cause a loss of efficiency because of the irreversible nature of the isenthalpic flash of the hotter liquid into the momentarily lower pressure space of the receiver into which it is being transferred. A small vapor space is necessary for mechanical reasons, however, to insure that no liquid water is introduced into the steam turbines, and the resultant small loss in efficiency must be tolerated as a safety factor. It should, however, be kept as small as practicable.

As it is desirable to have successive water displacements occur with a minimum of heat interchange, the interfacial contact areas of the receivers must be kept as small as possible. One practical design is a top and bottom header connected by relatively long tubes or small diameter pipes.

It should be noted that the thermal efiiciency of the waste heat boilers of the present invention is so great that often the best overall efliciency when used with a Brayton cycle gas turbine is maximized with a much smaller number of turbine stages than is normally desirable where there is not available such a highly efiicient waste heat system. In fact, in many cases a single stage gas turbine may be used, which of course saves on equipment which would otherwise be necessary for maximum turbine efficiency with multiple stages, reheats, and the like. This possibility of saving in the preferred modification of the present invention is of real practical importance.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows in purely diagrammatic form a flow sheet for recovering power from the exhaust of a Brayton cycle gas turbine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the figure, atmospheric air 1 enters compressor 2 of a Brayton cycle turbine, is compressed therein and directed to com'bustor 64, which is supplied with suitable fuel 65. The hot high pressure air, after being expanded and partially cooled in expander 63 of the Brayton cycle gas turbine, goes to heat recovery section 3 in which it gives up heat to superheaters 4, 5, and 6, and water heater 7, and is finally discharged cooled to atmosphere at 66-.

As the drawing is purely diagrammatic, only a single stage compressor 2, is shown. Of course this can be multistage and may be provided with cooling or refrigeration between stages. Since the present invention, even in its preferred embodiment, is not concerned with particular design details of the Brayton cycle from which the waste heat is obtained, the drawing is kept simple by omitting any constructional features which are not essential to the diagrammatic representation of a Brayton cycle plant. However, as referred to above, one of the advantages of the present invention when used with a Brayton cycle plant as a source of waste heat is that efficiency can be maximized with a much smaller number of turbine stages, preferably one, and this is what is illustrated in the drawing. To this extent it can be said that the waste heat system of the present invention can desirably modify the design of the Brayton cycle power plant.

Cold, low pressure water is pressurized by pump 13 to about 1500 p.s.i.a., heated in water heater 7 to about 596 F., and directed through surge tank 15 and valve 16 to high pressure receiver 17 of line A.

In line A water receivers 24 and 31 and condenser 11 operate at successively lower absolute pressures, which ideally at the start of the cycle should be such that Pump 18 circulates the water from receiver 17 through valve 19 to steam separator 20, in which the water flashes at progressively decreasing temperature and pressure as the flash vapor is removed. The flash vapor produced is directed via valve 21 through superheater 4 to the multiple nozzles of turbine 8, through which it expands to line 60, where it is joined by flash vapor similarly produced in steam separator 27 The combined vapor stream is heated in superheater 5 and flows to the multiple nozzles of turbine 9, through which it expands to line 61, where it is joined by flash vapor similarly produced in steam separator 34.

The combined vapor stream is again heated in superheater 6 and flows to the multiple nozzles of turbine 10, through which it expands to condenser 11. The shafts of turbines 8, '9 and 10 produce power, which is shown diagrammatically at 62. It can be any power absorbing element, such as a generator. Similarly, the power output of the turbine 63 of the Brayton cycle is shown connected to a power utilizing element 68. In the latter case, of course, the output power is the difference between the power produced and that absorbed by the compressor 2 which is driven by the turbine 63.

A second line of receivers and flash tanks, labeled B, is provided, with receivers 39, 46 and 53 corresponding to 17, 24 and 31 of line A; flash tanks 42, 49 and 56 corresponding to 20, 27 and 34, and pumps 40, 47 and 54 correpsonding to 18, 25 and 32. Valve connections in line B are 38, 43, 45, 50, 52, '57 and 55 and 59 respectively. Similarly, the check valves 44, 45 and 58 are provided.

When the pressure of receiver 17 has decayed to the initial pressure of receiver 24, whose pressure has decayed to the initial pressure of receiver 31, whose pressure has in turn decayed ideally to that of condenser 11, vapor valves 21, 28 and 35 are closed, and valves 43, 50 and 57 of steam separators 42, 49, and 56 of steam generating line B are simultaneously opened, thus providing continuity of motive steam supply to turbines 8, 9 and 10.

Pump valves 19, 26, and 33 are then closed, but pumps 18, 25 and 32 continue running on automatic recycle (not shown).

Water admission valves 16, 23 and 30 are then opened, pressurizing line A, and causing check valves 22, 29 and 36 to close, thus trapping the liquid and vapor contents of steam separators 20, 27 and 34 at the respective pres sures prevailing at the end of the cycle. It is necessary to have some residual vapor space in the separators in order to prevent liquid from entering the turbines when operations resume. However, the volumes of these spaces should be kept as small as possible, since they cause a loss in cycle efliciency, as will be explained later.

Water outlet valve 37 is then opened, permitting hot high pressure water to displace the lower temperature water contained in water receiver 17, which in turn displaces the water contained in water receiver 24, which displaces the water contained in water receiver 31.

The flow rate of water to surge drum is controlled by valve 67, which by regulating flow keeps the temperature of the water leaving heater 7 constant. The holding capacity of drum 15 must be suflicient to permit balancing the cyclic water requirements of vapor generating lines A and B with the substantially uniform flow rate to drum 15.

Valves 37, 30, 23 and 16 close when the liquid level of drum 15 falls a predetermined amount, corresponding to the cycle displacement volume. Pump valves 19, 26, and 33 are then reopened and line A is again ready to go on stream.

Line B operates in the same manner as has been described for line A, its corresponding pumps, valves, flash tanks and the like operating in the same manner, including the operation of check valves 44, 51 and 58 when the B line has contributed its power and is repressurized.

When pump valves 19, 26, and 33 are first reopened, the resulting irreversible isenthalpic flash of the hot liquids into the momentarily lower pressures prevailing in the residual vapor spaces of the stream separators causes the previously mentioned loss in cycle efficiency. The same phenomenon results when the valves 41, 48 and 55 are first reopened to put line B on stream.

Since it is desired to have the successive water displacements occur with a minimum mixing of the respective water streams, and with a minimum amount of heat interchange, the interfacial contact areas of the receivers must be kept as small as practicable. This is readily achieved by constructing the receivers 17, 24 and 31 of line A and 39, 46 and 53 of line B of top and bottom headers, connected by relatively long tubes or small diameter pipes, as shown schematically in the drawing.

The cycle timing is automatically set to keep the peak liquid level of drum 15 constant, thus equalizing in-flow and out-flow. In the case of line A multiple nozzles of turbine 8 are automatically operated to adjust the pressure decay rate of steam separator so as to conform both to this cycle timing and to the requirement for a substantially constant power production rate. The absolute pressure of steam separator 27 is automatically controlled at a desired percentage of the absolute pressure of separator 20 by the operation of the multiple nozzles of turbine 9, and in the same Way the desired absolute pressure relationship between separators 34 and 27 is automatically controlled by the operation of the nozzles of turbine 10.

The pressure of the steam produced in the steam separators decays from a high value at the start of a cycle to a low value at the end of a cycle, with a comparable reduction in weight flow through a given turbine nozzle passing steam at a given temperature through a given expansion ratio. In order to produce power at a substantially uniform rate, steam is admitted to a progressively larger number of nozzles as the steam pressure decreases.

The exhaust steam from turbine 8, cooled as a result of its expansion, joins the steam generated in separator 27 and is reheated in superheater 5. The same process occurs with the exhaust from turbine 9. The reheats improve the efliciency of the cycle.

In the case of line B the same operation of multiple nozzles in the turbines 8, 9 and 10 operate to adjust the pressure decay rate in steam separators 42, 49 and 56. Reheating of exhaust steam from turbines 8 and 9 proceeds in the same way as has just been described for the operation of the turbines from line A.

I claim:

1. A waste heat working fluid vapor generating system comprising,

(a) heat exchangers and a means to supply waste heat gases thereto, the heat exchangers including fluid heating elements and vapor superheating elements;

(b) a plurality of high pressure, high temperature liquid receivers in series parallel, the temperature and pressure in each receiver in each series progressively decreasing;

(c) means for pumping hot working liquid at a pressure above its vaporization point to the highest temperature liquid receiver of each series from at least one heat exchanger in the waste heat generator;

(d) multiple-stage vapor turbine means having a number of stages corresponding to the hot liquid receivers in each series;

(e) each turbine stage having a plurality of steam introducing nozzles;

(f) a series of flash chambers corresponding to the liquid receivers;

(g) control means for introducing hot pressurized liquid from the heat exchanger in the waste heat section to the highest pressure liquid receiver in each series;

(b) means for pumping liquid from the highest temperature liquid receiver to a flash chamber where it flash vaporizes through successively lower temperatures and pressures to a predetermined lower temperature and pressure;

(i) valved connecting means from the flash chamber to a vapor superheating heat exchanger in the waste heat section and thence to the nozzle sets in the high pressure section of the vapor turbine;

(j) means for sequentially and cumulatively opening the sets of nozzles as the vapor temperature and pressure in the flash chamber is reduced to maintain substantially uniform power production in the turbine stage;

(k) means for transferring liquid from the highest pressure receiver to the next pressure receiver and cutting off the pumping means to the high pressure flash chamber;

(1) pumping means for pumping liquid from the second high pressure liquid receiver to the second flash chamber where it flash vaporizes at progressively decreasing pressures to a predetermined level, valved means connecting said flash chamber to a second superheater exchanger in the waste heat section and thence to a set of sequenced nozzle sets in the next section of the vapor turbine and means for opening them sequentially and cumulatively as the vapor temperature and pressure decreases in the same manwhen the second series has flashed liquid down to ner as the nozzles in the high temperature section the predetermined temperature.

of the turbine; 2. A system according to claim 1 in which the working (m) the number of liquid receivers in series and the fluid is water.

number of turbine stages continuing to a point 5 3. A system according to claim 2 in which the waste at which liquid has been flashed into vapor at the heat is exhaust gas from the gas turbine of an open Brayton lowest predetermined pressure for the system, each cycle power generating system.

receiver being connected to its own flash chamber and its sequenced set of nozzles in the corresponding References Cited turbine stage in the same manner as in the preceding 10 UNITED STATES PATENTS Stages; 2,957,815 10/1960 Pacault et al 60-108 (n) means for connectlng the highest temperature 3,150,487 9/1964 Mangan et a1 B liquid receiver of a second series and cutting off vapor connections to superheaters from the flash chambers MARTIN SCHWADRON primary Examiner of the first series; 15 (0) means for repeating the cycles in the second series OSTRAGER Asslstant Exammer ofliquid receivers and flash chambers as in the first, CL

(p) means for repeating the cycle through the first series 60*? 104 

